1. Field of the Invention
The present invention relates to a voltage to current conversion circuit which, and more particularly a voltage to current conversion circuit for converting an input voltage to a multiple current outputs.
2. Description of the Prior Art
A circuit which converts an input voltage to a current, obtaining at the same time the absolute value of the voltage, is disclosed in Japanese Patent Application Laid-Open No. Sho 61-45314. FIG. 4 shows the circuit diagram of the absolute value voltage to current conversion circuit. In this absolute value voltage to current conversion circuit, one end of a resistor R6 is connected to an input voltage terminal Vin, and the other end of the resistor is connected in common to the inverting input of an operational amplifier A3, the base and the collector of an NPN transistor Q1, the base of an NPN transistor Q2, and the emitter of an NPN transistor Q3. The noninverting input of A3 is connected to a reference voltage terminal (the ground in this case). The output of A3 is connected in common to the emitter of Q1, the emitter of Q2, and the base of Q3. This conversion circuit is completed by connecting the collectors of Q2 and Q3 in common and serving it as an output terminal Io.
In this example, when the polarity of the input voltage Vin is positive, the transistors Q1 and Q2 go to active state and the transistor Q3 goes to cut-off state. This is because Q1 is in the active state so that the emitter and base junction of Q3 is in the reverse bias state. Since A3 is being subjected to a negative feedback through Q1, the inverting input of A3 goes to virtually grounded state so that a current given by Vin/R6 flows in Q1, on the assumption that the common emitter current amplification factor (hFE) of the transistor has such a large value that its effect can safely be neglected. Besides, the transistors Q1 and Q2 constitute a current mirror circuit, so that a current with the same value as that of the collector current of Q1 flows in the collector of Q2, which serves as the output current Iout. On the contrary, when the polarity of the input voltage Vin is negative, Q3 goes to the active state and Q1 and Q2 go to the cut-off state. Since A3 is subjected to a negative feedback via the base-emitter junction of Q3, the inverting input of A3 goes again to the virtually grounded state. Consequently, an output current Iout given by Vin/R6, the same expression as in the case of positive polarity of the input voltage Vin, is output from the output terminal Io via the collector of Q3.
To summarize the above, the relation between the input voltage Vin and the output current Iout is given by Eq. (1) below. EQU Iout.apprxeq..vertline.Vin.vertline./R6. (1)
In other words, the absolute value of an input voltage Vin is converted to a current by a resistance R6, and the relationship between the input voltage Vin and the output current Iout is as shown in FIG. 5. Here, it should be reminded that the result in Eq. (1) above is an approximation to the rigorous expressions given below. EQU Iout=(Vin+/R6)hFE/(1+hFE)!(for Vin&gt;0), (2) EQU Iout=(-Vin-/R6)hFE/(1+hFE)!(for Vin&lt;0). (3)
In the above, R6 is the resistance of the resistor R6, and hFE is the common emitter current amplification factor of the transistors Q1 to Q3.
In an NPN transistor formed as an integrated circuit, hFE generally has a value of more than 100 even for a small current on the order of several mA, although it depends on the emitter size. Accordingly, the conversion error due to hFE is less than 1% for both cases of positive and negative polarity of Vin, as can be seen from Eqs. (2) and (3), and the symmetry with respect to the polarity of the input voltage is excellent, justifying the validity of Eq. (1).
However, with the circuit in the above it is not possible to realize multiple output of the current. That is, owing to the nature of the current, if the current is taken once at a point as an output, then it is no longer possible to use it as another output at somewhere else. In contrast, in the case of the voltage, it is possible to take it out at a plurality of points as outputs so long as the output current capacity permits to do so.